TRPMLs and TPCs: Targets for lysosomal storage and neurodegenerative disease therapy?

Neurodegenerative diseases (ND) pose a serious health burden to society and healthcare systems alike, with increasing incidence rates especially within aging populations. Alzheimer's disease (AD) is the most prevalent type of ND or dementia, followed by Parkinson's disease (PD), multiple sclerosis, amyotrophic lateral sclerosis, and Huntington's disease. Progressive neurological dysfunction and regional neuronal loss constitute the common characteristics of ND. Many ND are accompanied by accumulation of protein aggregates such as extracellular amyloid-β (in AD), intraneuronal hyper-phosphorylated tau (in AD), or α-synuclein (in PD). Two main systems are responsible for the clearance of damaged, dysfunctional or senescent proteins inside cells: the autophagy-lysosomal pathway and the ubiquitin-proteasome system. The importance of lysosomes in neurodegenerative processes is further highlighted by clinical phenotypes of lysosomal storage disorders (LSDs), comprising more than 70 inheritable diseases caused by mutations in lysosomal enzymes or lysosomal membrane proteins, often resulting in severe neurodegeneration. Dysfunctional lysosomal proteins and enzymes result in the lysosomal accumulation of undigested macromolecules, e.g. lipids, glycoproteins, glycosaminoglycans, or gangliosides. Defects in intracellular transport pathways involving endosomes and lysosomes are increasingly recognized as drivers of neurodegenerative disease pathology including AD and PD. Thus, accumulation of damaged proteins and organelles (e.g. mitochondria) in neurons and glial cells overwhelms the capacity of intracellular recycling and degradation mechanisms, exacerbating disease pathology. Endolysosomal ion channels have recently been established as important regulators of lysosomal exocytosis, ion homeostasis/pH, endolysosomal trafficking, fusion and fission, and autophagy. In particular two non-selective endolysosomal cation channel families, the mucolipin/TRPML/MCOLN channels and the two-pore channels/TPCs will be discussed here as potential pharmacological targets for LSD/ND treatment.